Low Temperature Galvanomagnetic Properties of Graphites

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Abstract
The electrical resistances and Hall coefficients of polycrystalline graphite, neutron irradiated graphite, chemically doped graphite, and neutron irradiated chemically doped graphite have been measured over a range of magnetic fields at liquid nitrogen and liquid helium temperatures. The empirical equivalence of acceptor concentrations in irradiated graphite and in chemically doped graphite obtained by matching Hall coefficients has been found to be a function of the temperature of measurement. This observation may be explained in terms of temperature dependent trapping efficiencies of the electron traps introduced chemically or by neutron irradiation. This explanation affords some understanding of the electrical properties of the complicated neutron irradiated chemically doped graphite. The temperature variation of the resistances and Hall coefficients of the graphites studied may be reasonably well understood on the basis of the phenomenological theory of transport properties. Anomalous variations with magnetic field of resistances and Hall coefficients were observed at low temperature in some of the graphites studied; no satisfactory explanation has been found for these effects, although a recently introduced theory provides a plausible explanation for the observed magnetoresistance of polycrystalline graphite at liquid helium temperature.